hibernate批处理

在使用hibernate进行批处理的时候，需要考虑这两个问题：OutOfMemoryError、处理速度

解决方案：

<property name="hibernate.jdbc.batch_size">20</property>      <--使用JDBC batching-->
<property name="hibernate.cache.use_second_level_cache">false</property>     <--关闭二级缓存-->

使用session的flush（）和clear（）方法阶段性清除一级缓存

1.方法一：使用服务器游标

      1.1.批量插入

<span style="font-size:12px;">Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
   
for ( int i=0; i<100000; i++ ) {
    Customer customer = new Customer(.....);
    session.save(customer);
    if ( i % 20 == 0 ) {     //此处的值应和配置文件中的一致
        //强制内存到数据库同步，释放内存
        session.flush();
        session.clear();
    }
}
   
tx.commit();
session.close();</span>

      1.2.批量更新

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
   
ScrollableResults customers = session.getNamedQuery("GetCustomers")    //使用了外置命令查询
    .setCacheMode(CacheMode.IGNORE)
    .scroll(ScrollMode.FORWARD_ONLY);
int count=0;
while ( customers.next() ) {
    Customer customer = (Customer) customers.get(0);
    customer.updateStuff(...);
    if ( ++count % 20 == 0 ) {
        //<span style="font-size:12px;">强制内存到数据库同步，释放内存</span>
        session.flush();
        session.clear();
    }
}
   
tx.commit();
session.close();

2.方法二：使用StatelessSession接口

StatelessSession 不实现第一级 cache，也不和第二级缓存，或者查询缓存交互。它不实现事务化写，也不实现脏数据检查。没有第一级缓存。无状态 session 是低层的抽象，和低层 JDBC 相当接近。 

StatelessSession session = sessionFactory.openStatelessSession();
Transaction tx = session.beginTransaction();
   
ScrollableResults customers = session.getNamedQuery("GetCustomers")
    .scroll(ScrollMode.FORWARD_ONLY);
while ( customers.next() ) {
    Customer customer = (Customer) customers.get(0);
    customer.updateStuff(...);
    session.update(customer);
}
   
tx.commit();
session.close();

3 .方法三：使用hql

      3.1批量插入

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

String hqlInsert = "insert into DelinquentAccount (id, name) select c.id, c.name from Customer c where ...";
int createdEntities = s.createQuery( hqlInsert )
        .executeUpdate();
tx.commit();
session.close();

注意：使用hql插入操作 只支持 INSERT INTO ... SELECT ... 形式，不支持 INSERT INTO ... VALUES ... 形式。

            3.2批量更新

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlVersionedUpdate = "update versioned Customer set name = :newName where name = :oldName";
int updatedEntities = s.createQuery( hqlUpdate )
        .setString( "newName", newName )
        .setString( "oldName", oldName )
        .executeUpdate();
tx.commit();
session.close();